Current estimates of the US population suggest greater than 20% of men over 45 years of age may be hypogonadal (total Testosterone <300 pg/dL), and more than 30% of US men over 70 are considered hypogonadal. Over the last decade prescriptions for testosterone replacement therapy (TRT) have increased 3-fold. However, recent studies suggest that TRT can increase cardiovascular and cerebrovascular morbidity and mortality. The reasons underlying these risks are unknown. An understanding of the mechanism of action of T's effects on the cerebrovasculature and brain could lead to optimized therapies with respect not just to timing, but also dose, route, and formulation for TRT. Recent evidence indicates that there is a critical window for hormone therapy in postmenopausal women, beyond which estrogens lose their beneficial effects, and may even have detrimental effects on cognition and brain aging. Because many of T's effects in the brain involve conversion to estrogen and antioxidant effects similar to estrogen, we hypothesize that the beneficial effects of T in the brain may also have a critical window of efficacy in males. Further, oxidative stress levels may mediate this critical window. We have recently found that T levels are inversely related to cognitive function only under conditions of high oxidative stress in men. This PILOT grant seeks to investigate the concept of a critical window for TRT on cognition, brain aging, and response to oxidative injury. Morphological and behavioral studies will be used to define the role of dose and time-dependent steroid replacement regimens in a rat model. The overall goal is to establish a model of successful and refractory TRT that can then be used to generate mechanistic hypotheses for subsequent extramural grant submissions using natural aging and comorbid conditions in the context of TRT. Aim 1 will test the hypothesis that the beneficial effects of TRT on cognition and oxidative stress are diminished following long-term testosterone deprivation (LTTD). Castrate middle-aged rats will undergo short (1d) or long-term (10 wk) testosterone deprivation before replacement to physiological levels. Cognitive function and peripheral and central oxidative stress will be compared to intact and castrate rats treated with vehicle. Aim 2 will test the hypothesis that long-term testosterone deprivation (LTTD) inhibits the neuroprotective effects of TRT in experimental stroke. Castrate male middle-aged rats will undergo short (1d) or long-term (10 wk) testosterone deprivation before replacement to physiological levels. Rats will undergo experimental stroke using intraparenchymal injection of endothelin-1 adjacent to the middle cerebral artery. Sensory-motor function, cognitive function and peripheral and central oxidative stress will be compared to intact and castrate rats treated with vehicle. Results of these PILOT studies not only will provide critical preliminary data for additional mechanistic extramural grant proposals, but also will provide important information for those men considering TRT.